On Tue, Aug 19, 2014 at 03:10:55PM -0700, Zach Brown wrote:

On Sun, Aug 17, 2014 at 02:44:34PM +0200, Klaus Holler wrote:

quoted

Hello list,

I want to use an ARM kirkwood based NSA325v2 NAS (dubbed "Receiver") for
receiving btrfs snapshots done on several hosts, e.g. a Core Duo laptop
running kubuntu 14.04 LTS (dubbed "Source"), storing them on a 3TB WD
red disk (having GPT label, partitions created with parted).

But all the btrfs receive commands on 'Receiver' fail soon with e.g.:
  ERROR: writing to initrd.img-3.13.0-24-generic.original failed. File
too large
... and that stops reception/snapshot creation.

...

quoted

Increasing the verbosity with "-v -v" for btrfs receive shows the
following differences between receive operations on 'Receiver' and
'OtherHost', both of them using the identical inputfile
/boot/.snapshot/20140816-1310-boot_kernel3.16.0.btrfs-send

* the chown and chmod operations are different -> resulting in
weird/wrong permissions and sizes on 'Receiver' side.
* what's "stransid", this is the first line that differs

This is interesting, thanks for going to the trouble to show those
diffs.

That the commands and strings match up show us that the basic tlv header
chaining is working.  But the u64 attribute values are sometimes messed
up.  And messed up in a specific way.  A variable number of low order
bytes are magically appearing.

(gdb) print/x 11709972488
$2 = 0x2b9f80008
(gdb) print/x 178680
$3 = 0x2b9f8

(gdb) print/x 588032
$6 = 0x8f900
(gdb) print/x 2297
$7 = 0x8f9

Some light googling makes me think that the Marvell Kirkwood is not
friendly at all to unaligned accesses.

   ARM isn't in general -- it never has been, even 20 years ago in the
ARM3 days when I was writing code in ARM assembler. We've been bitten
by this before in btrfs (mkfs on ARM works, mounting it fails fast,
because userspace has a trap to fix unaligned accesses, and the kernel
doesn't).

The (biting tongue) send and receive code is playing some games with
casting aligned and unaligned pointers.  Maybe that's upsetting the arm
toolchain/kirkwood.

   Almost certainly the toolchain isn't identifying the unaligned
accesses, and thus building code that uses them causes stuff to break.

   There's a workaround for userspace that you can use to verify that
this is indeed the problem: echo 2 >/proc/cpu/alignment will tell the
kernel to fix up unaligned accesses initiated in userspace. It's a
performance killer, but it should serve to identify whether the
problem is actually this.

   Hugo.

-- 
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